A personal comfort heater cable that practically does not radiate electromagnetic and electrostatic fields is described in U.S. Pat. No. 4,910,391 assigned to the same assignee as for this patent application. As described in one embodiment in this patent, a foil is wrapped around an insulator and a central conductor to reduce electromagnetic and electrostatic radiation.
Although a foil is effective in reducing such radiation, it is not always easy to handle and complicates the manufacture of the personal comfort heater cable.
Coaxial-type heater cables for personal comfort products have been described in the art. For example, in U.S. Pat. No. 4,661,690 to Yamamoto et al, helically-wrapped inner and outer conductors with an interposed PTC material are shown and described. Other coaxial heater wires are shown in U.S. Pat. Nos. 4,503,322 and 4,575,620.
U.S. Pat. No. 4,757,297 to Frawley shows an ignition cable in which a twisted central multiple conductor is enclosed by a semiconductor layer having sufficient resistance to dampen high frequency currents on the surface of the center conductor. An optional metal braid and insulator enclose the semiconductor layer and central conductor.
U.S. Pat. No. 4,143,238 to Sheth shows and describes a low voltage miniature cable containing one or a plurality of parallel insulated conductors that are surrounded by a semiconductor coating to provide a ground shield.
U.S. Pat. No. 4,008,367 shows and describes a power cable in which an outer metal shield encloses a plurality of inner layers, one of which is a conductive polymer layer. Another similar high voltage shielding construction is shown in U.S. Pat. No. 3,049,584.
U.S. Pat. No. 3,836,482 describes electric cables having semiconducting outer jackets in which drain wires are embedded (see also U.S. Pat. No. 3,474,189) to provide both shielding and mechanical protection.
Semiconductive shielding compounds are commercially available, such as Union Carbide's polyethylene DHDA-7704 Black 55. Such compound is intended to be extruded over strand and insulation shielding on thermoplastic insulated high voltage cables and over crosslinked polyethylen insulated high voltage cables.